Multicolor printers produce images which are composed of a plurality of different single-color images. The quality of the final multicolor image depends, i.a., on the registration accuracy of the single-color images. With the increasing resolution of modern printers the registration accuracy has become an issue of interest.
Different multicolor printer types are known. Ink-jet printers have at least one print head from which droplets of ink are directed towards a print medium. Within the print head the ink is contained in a plurality of channels. Pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels. These pulses are generated e.g. by thermal components in thermal ink-jet print heads or by piezo-electric elements in drop-on-demand print heads. Ink-jet printers of the carriage type have a print head for each color. The print heads are mounted on a reciprocating carriage. Full-width or page-width ink-jet printers have, for each color, an array of nozzles extending across the full width of the print medium which is moved past the nozzle arrays. Each nozzle array is part of a print station which forms one single-color image or a part of it. Each print station produces its own single-color image on the print medium as it moves past the print stations. Each single-color image is composed of a plurality of closely spaced image dots, wherein single-color dots are superimposed to form a dot of a required color. The superimposed single-color dots may be printed onto each other or in a side-by-side relation. The recording medium may be paper or any other suitable substrate to which the ink adheres.
In known color xerographic systems, instead of the nozzle arrays, a plurality of print bars are provided which produce an electrostatic charge image on a recording medium. The print bars are selectively energized to create successive charge images, one for each color. The print bars may, for example, be LED print bars which produce the charge image an a previously charged photoreceptive surface. Each LED print bar is associated with a development system, which develops a latent image of the last charge image or exposure without disturbing previously developed images. The fully developed color image is then transferred to an output sheet, e.g. paper or the like. It is also possible to form electrostatic charge images directly on the output sheet which is then exposed to a toner of the respective color to produce a visible image.
To register single-color images for forming a multicolor image, encoder arrangements are utilized which determine the advance of the recording medium during the print process. Optical encoder systems are known in which an optical sensor is responsive to encoder marks.
In page-width printers the recording medium is, for example, moved by a conveying belt which is driven by rollers or pulleys. The movement of the belt with the recording medium may be detected by a single rotary encoder which is mounted on one of the rollers or pulleys. The advance of the belt is controlled by advance information represented by the rotary encoder signals. It is also known to place the encoder marks on the belt.
U.S. Pat. No. 5,526,107 is directed to a system and method for duplex printing wherein two images are registered at corresponding locations on the two sides of a print medium. The positions of the printed images relative to each other are synchronized by mechanical means or by detecting the position of special marks or an area of the image itself. It is mentioned that color-to-color registration may be achieved using a similar synchronization technique.
U.S. Pat. No. 4,804,979 discloses an electrostatic color printer with several print stations. Encoding marks are printed on the recording medium (which is paper). Each print station has its own optical sensor responsive to the encoding marks to detect and correct for variations of the recording medium to obtain registration of the single-color images. The registration is checked every 50 raster lines and brought into exact registration, if necessary.
EP 0 729 846 B1 discloses a high-speed ink-jet printing press in which registration marks are printed on the print medium. The registration marks are used, at low speeds, for aligning the recording stations, and, at high speeds, for registering the single-color images.